In the standard model there is a source of CP violation from the phase in the Kobayashi-Maskawa mixing
matrix. This, however, is not large enough to explain the baryon asymmetry observed in nature.
Another source is the phase in the interaction between gluons (\(\theta \) -parameter), naively expected to be of order unity.
The non observation, however, of elementary electron dipole moment, set limits its value to be \(\theta \le 10^{-9}\) .
This has been known as the strong CP problem.
A solution to this problem has been the P-Q (Peccei-Quinn) mechanism.
In extensions of the Standard Model (S-M), e.g. two Higgs doublets, the Lagrangian has a global
P-Q chiral symmetry \(U_{PQ}(1)\) , which is spontaneously broken, generating a Goldstone boson, the
axion (a). In fact
the axion has been proposed a long time ago as a solution to the strong CP problem [1]}
resulting to a pseudo Goldstone Boson [2]}, [3]}. The two most widely cited models
of invisible axions are the KSVZ (Kim, Shifman, Vainshtein and Zakharov) or hadronic axion models
[4]}, [5]}
and the DFSZ (Dine,Fischler,Srednicki and Zhitnitskij) or GUT
axion model [6]}, [7]}.
This also led to the interesting scenario of the axion being a candidate for dark matter in the
universe [8]}, [9]}, [10]} and it can be searched for by real
experiments [11]}, [12]}, [13]}, [14]}. For a recent review see [15]}.
The relevant phenomenology has also been reviewed [16]}.
